The formation of “whiskers” in Sn (tin) films is a subject of great research interest. Many electronic devices have an anticipated operational lifetime of ten years or more. Yet whiskers may form in a period of months or less. Some whiskers have been observed to reach lengths of several millimeters. Whiskers such as these present a significant reliability risk in electronic systems due to the possibility of forming an undesired conductive path (a short) or causing mechanical damage to particle sensitive devices such as disk drives.
It has long been known that alloying Sn with lead (Pb) reduces the formation of Sn whiskers. However, environmental concerns over the adverse ecological effects of Pb have resulted in legislation in various parts of the world which explicitly restricts the use of Pb in electronics. For example, the Restriction of Hazardous Substances (RoHS) directive, adopted by the European Union in 2003 and effective beginning in 2006, restricts the concentration of lead in any homogeneous component to 0.1% (1000 ppm) or less. The permitted concentration of Pb is, in general, too low to suppress formation of Sn whiskers in all cases in which such whiskers present an unacceptable reliability risk.
Formation of Sn whiskers remains poorly understood. Current theories offer insufficient guidance to provide acceptable alternatives to Pb-doping that are environmentally benign and low-cost.